1. Field of the Invention
The present invention relates to a method of fabricating a photomask, and more particularly, to a method of repairing a defect in a photomask.
2. Description of the Related Art
Photolithography is a critical process in the fabrication of integrated circuits. In particular, today's very high integration level of integrated circuit devices demands a highly reliable photolithography which employs a photomask that can define extremely fine patterns and that is defect-free. Typical pattern defects of a photomask include a "clear" defect and an "opaque" defect. The clear defect represents damage to a part of the pattern. The opaque defect occurs when debris of the light shielding or light phase-shifting material that forms the pattern is present on a light transmitting portion of the photomask.
The conventional method for repairing an opaque defect involves removing the debris of the pattern material by ion beam etching. However, this method frequently results in an undesirable recessed region R being formed on the mask substrate 10, as shown in FIG. 1, which shows a cross-section of a mask after an opaque defect in it has been repaired by the conventional ion beam etching repair process. The recessed region R, which is deeper than other portions of the substrate 10, is formed generally along the perimeter of the opaque defect, since the etch rate of the substrate 10 is substantially similar to that of the material forming the opaque defect. Reference numeral 12 refers to the mask pattern. In addition to the recessed region R, the conventional ion beam etching repair also produces "ion stains" around the repaired portion of substrate 10. The extra etching step necessary to remove the ion stains makes the recessed region R even deeper. A large step difference d, and the recessed region R of the substrate 10, cause a phase difference between the light transmitted by the normal portion of the substrate 10 and the light transmitted by the recessed region R of the substrate 10, thereby producing an error in the pattern reproduced on the work piece. Therefore, photolithography using a photomask that has been repaired by the conventional method can result in the production of defective parts.